A Scanning Probe Microscope (SPM) refers to a microscope that measures surface characteristics of a sample so as to be shown as a 3D image while allowing a nanoprobe manufactured by a MEMS process to scan over a surface of the sample (scanning). The SPM is subdivided into an Atomic Force Microscope (AFM), a Scanning Tunneling Microscope (STM), and the like depending on a measurement method.
Generally, the SPM such as the AFM uses an optical vision system in order to determine a measurement position of a probe. The optical vision system includes a digial camera using an image sensor, such as a charged-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS), an object lens arranged toward a surface side of a target to be measured, and a body tube. Such body tube is configured to optically connect the camera with the object lens and transfer an image falling on the object lens to the image sensor of the camera.
When a surface of the target is measured using the SPM, the above-described optical vision system is moved in a Z-direction (up and down direction) using a precision stage so as to be focused on the surface of the target. Therefore, an image of the surface of the target falls on the image sensor, and a signal output from the camera is displayed on a display device such as a monitor. Thus, a user can observe the surface of the target, which is enlarged according to a magnification of the object lens.
Then, while the surface of the target is observed using the optical vision system, the target is moved to a desired position using an XY-stage configured to move the target in X and Y directions. Then, the target is measured by the SPM.
Such a SPM, particularly an AFM, has often been used for a defect review of a flat target to be measured such as a wafer. Such a defect review is conducted by checking a position of a defect using an optical vision system, moving a probe (actually moving the target) to the position, and imaging a detailed shape of the defect through the probe.
However, if the defect of a flat surface of the target to be measured such as a wafer has a small width, it is difficult to observe the presence of the defect with the optical vision system. Therefore, a defect review cannot be conducted or may be restrictively conducted by the SPM in many cases.
FIG. 1a is a surface image of a wafer observed with an optical vision system. FIG. 1b is a background image acquired by image-processing the surface image of FIG. 1a using a low pass filter. FIG. 1c is an image acquired by subtracting the background image from the image of FIG. 1a. FIG. 1d is an image acquired by overlapping the image of FIG. 1c 32 times, and FIG. 1e is an image acquired by overlapping the image of FIG. 1c 512 times.
Referring to FIG. 1a, it is difficult to find a defect on the wafer with the naked eye. Therefore, typically, a high-frequency noise component is removed using a low pass filter so as to acquire the background image as shown in FIG. 1b. Then, the background image is removed from the image of FIG. 1a so as to acquire the image as shown in FIG. 1c. However, referring to FIG. 1c, it can be seen that even in this case, it is difficult to observe defects indicated by arrows with the naked eye.
Accordingly, in order to achieve an averaging effect, the image of FIG. 1c is overlapped multiple times (32 times) so as to acquire the image of FIG. 1d. Since a signal-to-noise ratio (SNR) is proportional to the square root of the number of times of overlapping, if the image is repeatedly overlapped, the defects are clearly visible compared to the noise. Therefore, a defect indicated by an arrow in FIG. 1d is more clearly visible, but any defect smaller than this defect is still invisible.
Further, referring to FIG. 1e, such overlapping has a marginal effect. Thus, even when the image is overlapped 512 times, a defect is not more clearly visible than the defect in the image overlapped 32 times.
Therefore, even if an image is overlapped, there is a limit in observing a defect with an optical vision system. Thus, a defect, to which a defect review needs to be conducted may often be overlooked. It is considered that such a problem is caused by non-uniformity between pixels of an image sensor, by which non-uniformity is caused depending on a position of an optical system.